AGENTS.md, README: address second round of MR review

AGENTS.md, README: address second round of MR review

- Golden rule #1: note license texts are a second WHENCE-accounting
  exception (referenced via Licence: lines, stored in LICENSES/), not
  just repository tooling in the known_files allowlist.
- Require new license files to live in the LICENSES/ directory, in both
  AGENTS.md and README.md, matching the LICENSES/ layout now on main.
- Encourage including a firmware changelog in the commit message, since
  for binary blobs the commit message is often the only human-readable
  record of the change.

Signed-off-by: Josh Boyer <jwboyer@kernel.org>
Assisted-by: Claude Opus 4.8 <noreply@anthropic.com>